Several security related problems have been discovered in Mozilla
which are also present in Mozilla Thunderbird. The Common
Vulnerabilities and Exposures project identifies the following
vulnerabilities:

Paul Nickerson discovered that content-defined setters on an
object prototype were getting called by privileged user interface
code, and "moz_bug_r_a4" demonstrated that the higher privilege
level could be passed along to the content-defined attack code.
[MFSA-2006-37]

Mozilla team members discovered several crashes during testing of
the browser engine showing evidence of memory corruption which may
also lead to the execution of arbitrary code. This problem has
only partially been corrected. [MFSA-2006-32]

Chuck McAuley discovered that a text input box can be pre-filled
with a filename and then turned into a file-upload control,
allowing a malicious website to steal any local file whose name
they can guess. [MFSA-2006-41, MFSA-2006-23, CVE-2006-1729]

Masatoshi Kimura discovered that the Unicode Byte-order-Mark (BOM)
is stripped from UTF-8 pages during the conversion to Unicode
before the parser sees the web page, which allows remote attackers
to conduct cross-site scripting (XSS) attacks. [MFSA-2006-42]

Kazuho Oku discovered that Mozilla's lenient handling of HTTP
header syntax may allow remote attackers to trick the browser to
interpret certain responses as if they were responses from two
different sites. [MFSA-2006-33]